I do not want the distorted sound. please help!

I just recieved a new Moog standard Theremin.The first weekend it worked out very well, but the other night when I tried to play the tone was fine the first 30 sec and then it became more and more distorted and somehow darker, and after a while it is a completely different sound (not aÂ´the pure "sinus tone" at all!). I tried again yesterday and today and the same thing happened. I also tried to play on it in different rooms/places to see if there was a disturbing radio nearby, but this seems not to be the case. The range of pitch is also much smaller than before. This makes me vary sad. What should I do?

Had the same phenomenon some days ago. Moving the theremin around in the house, including one level higher I discovered that the new xmas illumination in the street in front of the house did these negative effects (perhaps a switching power supply).

That sounds like a problem with a coupling capacitor somewhere in the circuit. This is the sort of failure mode that occurs when an electrolytic capacitor is going bad.

If you are brave enough to dive inside of your theremin, you can look for the aluminum electrolytic capacitors and try changing them one-by-one. Just be sure to get the same or higher voltage rating and to align the + side with the + marking on the board. (I'm guessing you don't have an oscilloscope to go poking about in the circuit to determine which one might be bad).